Chore: Replace go-gomail/gomail with wneessen/go-mail

[wneessen]

I've noticed that diun is using https://github.com/go-gomail/gomail for sending mails via SMTP. go-mail/gomail is not actively maintained anymore and hasn't seen an update/response to an issue by the maintainer/merged PR in more than 6-7 years.

I am the maintainer of https://github.com/wneessen/go-mail which is pretty similar to go-mail/gomail but actively maintained and following modern standards and idiomatic Go. If there is any interest, I am willing to replace go-gomail/gomail in diun with wneessen/go-mail.

Let me know if a PR is of interest.

[crazy-max]

Sure, happy to have it replaced! Thanks!